A scanning electron microscope (SEM), a focused ion beam (FIB) processing device, or a device that uses other types of converged charged particle beams (i.e., probe beams) observes an image of a sample and processes the sample by scanning the surface of the sample with a probe. The resolution and the processing accuracy of such charged particle beam device are determined by the size of the probe cross-section (i.e., probe diameter). In principle, the smaller the probe diameter, the higher the resolution and processing accuracy that can be achieved.
In recent years, aberration correctors for charged particle beam devices have been developed and put into practical use. For an aberration corrector, multiple stages of multipole lenses having magnetic poles or electrodes are used. In each stage, a non-rotationally symmetrical electric field or magnetic field, such as a dipole field, a quadrupole field, a hexapole field, or an octupole field, is applied to a beam in a manner superimposed thereon, so as to provide an inverse aberration to the probe beam. Accordingly, the aberration corrector cancels out a variety of aberrations, such as spherical aberrations and chromatic aberrations generated on an objective lens, a polarizing lens, or the like of an optical unit.
However, as the aberration corrector needs a large number of power supplies for the multiple poles, complicated adjustment operations are required. Thus, attempts have been made to automate aberration correction by quantifying the aberration amount of an optical unit and feeding back an inverse aberration amount thereof to the charged particle beam device (for example, Patent Literature 1).
Typically, there is a plurality of types of aberrations including third-order and lower-order aberrations. However, a field that is necessary to correct each aberration is not independent of each other. Therefore, when some type of aberration is reduced, other types of aberrations can increase. Thus, it is typically necessary to repeat feedback a plurality of times to gradually optimize all aberrations. In this specification, a series of operations (i.e., one cycle) from “measurement of an aberration” to “reflection of the measurement result into a power supply value of an aberration corrector” will be referred to as “performing aberration correction.”
In the actual charged particle beam device, an aberration of an objective lens is corrected using an aberration corrector. However, as an aberration occurs due to a positional deviation of each pole of the multipole lens in the aberration corrector, variations in the magnetic properties of polar materials, and the like, there may be cases where a distribution of the generated field would deviate from the ideal field distribution of the multiple poles even when the electric field or magnetic field of the multipole lens is controlled. This in turn may generate a lower-order field, such as a dipole field or a quadrupole field.
When a charged particle beam enters a multipole lens that has a field distribution deviation with respect to the charged particle beam for some reason, the charged particle beam would be influenced by a dipole field or quadrupole field originating from the deviation, whereby the trajectory of the beam would also deviate. Consequently, an axis deviation, out-of-focus, or the like would occur, which could influence the resulting image quality. As described above, a lower-order field that is incidentally generated upon occurrence of a deviation from the ideal field when the electric field or magnetic field of an aberration corrector is changed, in particular, will be collectively referred to as a “parasitic aberration.” In the following description, the term “aberration,” when used alone, refers to a spherical aberration or chromatic aberration of an objective lens, and is distinguished from a “parasitic aberration” that is generated in the corrector.
The “parasitic aberration” appears as a lower-order field than the field of the multiple poles that should be originally controlled. A parasitic dipole field or a parasitic quadrupole field that can cause an axis deviation or out-of-focus, in particular, has a large influence on the resulting image. Thus, when the field of the multiple poles is changed, it would be necessary to cancel out the influence of the change by superimposing a dipole field or a quadrupole field on the field.
A parasitic aberration occurs when a generated field deviates from an ideal field. Therefore, it would be difficult to take a measure by predicting a deviation that may occur through simulation or the like before producing an aberration corrector. Accordingly, an operator should manually operate the aberration corrector and perform adjustment while checking an image of an axis deviation or out-of-focus that has occurred after changing the field of the multiple poles.
Meanwhile, in order to automate aberration correction, it is required to minimize parasitic aberrations that would be generated when the field of the multiple poles is changed in the automatic aberration correction sequence. This is because, if a charged particle beam does not reach the surface of the sample due to a significant axis deviation originating from parasitic aberrations and acquisition of an image has thus failed, or if a big out-of-focus has occurred and the image quality has thus deteriorated to such an extent that the sample cannot be identified as a result of changing the field of the multiple poles, it would be impossible to compute the magnitude of an aberration from the image and execute a continuous aberration correction sequence.
In order to address such problem, it would be necessary to move the field of the multiple poles in the actual device after producing the aberration corrector and inspect in advance a parasitic aberration that occurs at that time, and set information on the amount of a dipole field or a quadrupole field that should be adjusted to correct the aberration, on the device in advance.
A technique related to a method of correcting parasitic aberrations is disclosed in Patent Literature 2. Patent Literature 2 discloses a method of correcting a parasitic dipole field and a parasitic quadrupole field that are generated due to mechanical/electrical deviations of multiple poles.